Bilinear noise subtraction at the GEO 600 observatory

We develop a scheme to subtract off bilinear noise from the gravitational wave strain data and demonstrate it at the GEO 600 observatory. Modulations caused by test mass misalignments on longitudinal control signals are observed to have a broadband effect on the mid-frequency detector sensitivity ranging from 50 Hz to 500 Hz. We estimate this bilinear coupling by making use of narrow-band signal injections that are already in place for noise projection purposes. A coherent bilinear signal is constructed by a two-stage system identification process where the involved couplings are approximated in terms of stable rational functions. The time-domain filtering efficiency is observed to depend upon the system identification process especially when the involved transfer functions cover a large dynamic range and have multiple resonant features. We improve upon the existing filter design techniques by employing a Bayesian adaptive directed search strategy that optimizes across the several key parameters that affect the accuracy of the estimated model. The resulting post-offline subtraction leads to a suppression of modulation side-bands around the calibration lines along with a broadband reduction of the mid-frequency noise floor. The filter coefficients are updated periodically to account for any non-stationarities that can arise within the coupling. The observed increase in the astrophysical range and a reduction in the occurrence of non-astrophysical transients suggest that the above method is a viable data cleaning technique for current and future gravitational wave observatories

First demonstration of 6 dB quantum noise reduction in a kilometer scale gravitational wave observatory

Photon shot noise, arising from the quantum-mechanical nature of the light, currently limits the sensitivity of all the gravitational wave observatories at frequencies above one kilohertz. We report a successful application of squeezed vacuum states of light at the GEO\,600 observatory and demonstrate for the first time a reduction of quantum noise up to $6.03 \pm 0.02$ dB in a kilometer-scale interferometer. This is equivalent at high frequencies to increasing the laser power circulating in the interferometer by a factor of four. Achieving this milestone, a key goal for the upgrades of the advanced detectors, required a better understanding of the noise sources and losses, and implementation of robust control schemes to mitigate their contributions. In particular, we address the optical losses from beam propagation, phase noise from the squeezing ellipse, and backscattered light from the squeezed light source. The expertise gained from this work carried out at GEO 600 provides insight towards the implementation of 10 dB of squeezing envisioned for third-generation gravitational wave detectors

Direct limits for scalar field dark matter from a gravitational-wave detector

The nature of dark matter remains unknown to date; several candidate particles are being considered in a dynamically changing research landscape. Scalar field dark matter is a prominent option that is being explored with precision instruments, such as atomic clocks and optical cavities. Here we report on the first direct search for scalar field dark matter utilising a gravitational-wave detector, which operates beyond the quantum shot-noise limit. We set new upper limits for the coupling constants of scalar field dark matter as a function of its mass, by excluding the presence of signals that would be produced through the direct coupling of this dark matter to the beamsplitter of the GEO$\,$600 interferometer. The new constraints improve upon bounds from previous direct searches by more than six orders of magnitude, and are in some cases more stringent than limits obtained in tests of the equivalence principle by up to four orders of magnitude. Our work demonstrates that scalar field dark matter can be probed or constrained with direct searches using gravitational-wave detectors, and highlights the potential of quantum-enhanced interferometry for dark matter detection

Direct limits for scalar field dark matter from a gravitational-wave detector

The nature of dark matter remains unknown to date, although several candidate particles are being considered in a dynamically changing research landscape1. Scalar field dark matter is a prominent option that is being explored with precision instruments, such as atomic clocks and optical cavities2–8. Here we describe a direct search for scalar field dark matter using a gravitational-wave detector, which operates beyond the quantum shot-noise limit. We set new upper limits on the coupling constants of scalar field dark matter as a function of its mass, by excluding the presence of signals that would be produced through the direct coupling of this dark matter to the beam splitter of the GEO600 interferometer. These constraints improve on bounds from previous direct searches by more than six orders of magnitude and are, in some cases, more stringent than limits obtained in tests of the equivalence principle by up to four orders of magnitude. Our work demonstrates that scalar field dark matter can be investigated or constrained with direct searches using gravitational-wave detectors and highlights the potential of quantum-enhanced interferometry for dark matter detection. © 2021, The Author(s)

Développement de systèmes de contrôle in situ des propriétés optiques de filtres interférentiels

The FRESNEL Institute’s Research Team on Optical Thin Films (RCMO) owns a set of vacuum deposition machines using technologies based on physical vapor deposition (PVD). The realization of complex filtering functions requires a perfect mastering of the deposition process as well as accurate real-time monitoring of the optical thickness of the deposited layers. There are different monitoring techniques based on physical or opticalmethods, the principle consisting in the latter case to follow the evolution of the spectral performance of the stack during its formation. As an example, we can cite the monitoring methods called Monochromatic or Broadband. During my Ph.D. thesis, devoted to the development of new methods of in situ optical monitoring, I was particularly interested in two different subjects, namely :- On the one hand, the determination of the spectral dependence of optical constants (refractive index and extinction coefficient) of dielectric materials. Indeed, the knowledge of these constants is a key issue for the manufacturing of high-performance optical filters and a possible way to achieve this determination consists of using a broadband optical monitoring system in order to record the transmission spectra, in real-time, of the stack during its formation. Indeed, the temporal evolution, at each wavelength, of these transmission spectra provide quantitative information related to the optical constants that we wish to determine. I therefore theoretically established the mathematical nature of this relation, and applied this method to determine the optical constants of tantala (Ta2O5) and silica (SiO2) layers deposited through a Dual Ion Beam Sputtering (DIBS), and this without the use of index dispersion models.- On the other hand, the real-time measurement of the reflection coefficient (r) of a stack, in amplitude and phase, during its deposition. Indeed, the optical monitoring methods based on intensity properties present some limitations that the knowledge of phase information should overcome. This measurement is performed by low coherence digital holographic interferometry on a substrate illuminated by its rear face and whose front face is equipped with an annular mask. This gives access to desired phase and amplitude information while avoiding the parasitic influence of the substrate motions induced by the vibrations of the deposition machine, and therotation of the substrate holder at 120 rounds per minute. This method allows us to perform a phase mapping of the central zone of the substrate during the deposition runs of high and low index materials. Obviously, this method can be extended to the monitoring of more complex stacks.L’équipe de Recherche en Couches Minces Optiques (RCMO) de l’Institut FRESNEL dispose d’un ensemble de machines de dépôt sous vide utilisant le dépôt physique en phase vapeur. La réalisation de fonctions de filtrage complexes nécessite une parfaite maîtrise du processus de dépôt ainsi qu’un contrôle précis et en temps réel de l’épaisseur optique des couches déposées. Il existe différentes techniques de contrôle basées sur des méthodesphysiques ou optiques, le principe consistant dans ce dernier cas à suivre l’évolution des performances spectrales de l’empilement au cours de sa formation. À titre d’exemple, nous pouvons citer ici les méthodes de contrôle dites Monochromatique ou Large Bande. Au cours de ma thèse, consacrée au développement de nouvelles modalités de contrôle optique in situ, je me suis particulièrement intéressé à deux sujets différents, à savoir :- D’une part, la détermination de la dépendance spectrale des constantes optiques (indice de réfraction et coefficient d’extinction) de matériaux diélectriques. En effet, la connaissance de ces constantes est capitale si l’on souhaite réaliser des filtres optiques de hautes performances, et un moyen possible pour effectuer cette détermination consiste à utiliser un système de contrôle optique large bande afin d’enregistrerles spectres de transmission de l’empilement au fur et à mesure de sa formation. En effet, l’évolution temporelle, à chaque longueur d’onde, de ces spectres de transmission contient des informations quantitatives liées aux constantes optiques que nous souhaitons déterminer. J’ai donc établi de manière théorique la nature mathématique de cette relation et appliqué cette méthode à la détermination des constantes optiques de couches de pentoxyde de tantale (Ta2O5) et de dioxyde de silicium (SiO2) déposées par pulvérisation ionique assistée, et ce, sans avoir recours à l’utilisation de modèles de dispersion d’indice.- D’autre part, la mesure en temps réel du coefficient de réflexion (r) d’un empilement, en amplitude et en phase, lors de son dépôt. En effet, les méthodes de contrôles optiques en intensité présentent des limitations que la connaissance de l’information de phase devrait permettre de contourner. Cette mesure est réalisée par interférométrie holographique digitale à faible cohérence sur un substrat éclairé par sa face arrière et dont la face avant est équipée d’un masque annulaire. Ceci donne accès aux information de phase et d’amplitude recherchées tout en s’affranchissant des vibrations générées par le fonctionnement de la machine de dépôt ainsi que du mouvement de rotation à 120 tours par minute qu’effectue le porte-substrat. Cette méthode nous a permis de réaliser des cartographies de phase sur la zone centrale du substrat pendant la construction de couches haut et bas indice, la méthode étant évidemment extrapolable au contrôle et au suivi d’empilements plus complexes